Yup, things like budget, interests etc would help to narrow the choice down a little. Good advice from koolbreez, play it by ear. October is low season (ie cheap and plentiful accommodation) and also just when the monsoon changes ....well according to "official wisdom", weather seems to be getting increasingly unpredictable recently... so it's hard to say whether it'll be sunnier on the Andaman side or the Gulf side. You need to be flexible so that you aren't stuck on a beach where it's very damp.
I've noticed a few times when flying over Thailand in October that there are fewer clouds in north, so maybe think about doing Chiang Mai etc first.

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“Not all those who wander are lost.” Tolkien.
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